It has heretofore been known that the thermal stability and barrier properties of oriented blow molded containers of polyethylene terephthalate are significantly increased by heat setting. Typical processes for heat setting are shown in U.S. Pat. Nos. 4,476,170, 4,512,948 and 4,522,779.
In U.S. Pat. Nos. 4,476,170 and 4,512,948, there are disclosed an article and a process of making an oriented and heat set blow molded container of polyethylene terephthalate. In the process, a preform preheated to a temperature suitable for orientation is biaxially stretched in a blow mold and then while the hollow container is still in contact with the blow mold walls, the article is raised to a higher heat setting temperature preferably in the range of 200.degree.-250.degree. C. (except for the neck) thus hear setting the container, and while the container is still at a shrinkage resisting pressure exceeding atmospheric, it is cooled in the same mold to a temperature at which it maintains its shape when not pressurized but not below 100.degree. C. It is also disclosed that this cooling step can be done in the air outside the mold while maintaining internal pressure. According to these patents, when the heat setting temperature of the hot mold ranges from 220-250.degree. C. and the quenching temperature is not below 100.degree. C., higher onset-of-shrinkage temperatures are obtained.
In U.S. Pat. No. 4,522,779, there are disclosed improved plastic containers and a process for their production. In the first embodiment, a container is blow molded in a first hot blow mold, then reblown to a larger size in a second cold mold of larger volume than the first hot mold. Such containers are stated as having improved mechanical properties, particularly very high hoop yield stresses. However, the utilization of a larger volume cold mold substantially reduces the thermal stability. In the second embodiment, a container is blow molded in a hot blow mold, then reblown to a larger size in a second hot blow mold where it is blown to the confines of the second mold and the container is then removed from the second hot mold and transferred to a third cold mold and cooled to room temperature while maintaining internal pressure. In a further embodiment, the container is blow molded in a first hot mold, reblown in a second hot mold, and thereafter the second mold is cooled to cool the container.
U.S. Pat. No. 4,385,089 (British Patent Specification 1,604,203) is directed to heat set biaxially oriented hollow articles and states that the preform or parison should be heated at least to biaxially oriented temperature and maintained in closed contact with a hot mold which is at a temperature of up to 40.degree. C. above the minimum orientation temperature. In one embodiment, the resultant molded hollow article is moderately cooled causing a temperature drop of 10-30.degree. C. by introducing cooling vapor or mist into the hollow article, interrupting the cooling vapor, and opening the mold. In another embodiment, the heat set article is allowed to shrink freely and then reblown in the same hot mold or in a separate cooled mold. The patent calls for a heat setting temperature of 40.degree. C. above the orientation temperature which limits thermal stability and barrier properties.
According to this patent, the temperature of the hot mold should be maintained between 30 and 50.degree. C. above the minimum orientation temperature of the plastic material. Otherwise, it is stated there are numerous disadvantages including lowering of the production rate, the danger of the appearance of major distortion and major shrinkage on mold re-ease, the disadvantage inherent in heating metal molds to very high temperatures and keeping them at such temperature, and the danger of crystallization which would cause a loss of transparency. Further, in accordance with this prior patent, excessive shrinkage is to be avoided and generally the temperature drop of 10 to 30.degree. C. should be made. Accordingly, such a method precludes obtaining a degree of heat setting which would produce thermal stability at higher temperature as may be required in filling the container with various products. In addition, such a method will preclude obtaining the higher degrees of crystallinity and resultant high barrier properties which are required for some products.
In copending application Ser. No. 923,503 filed Oct. 27, 1986, having a common assignee with the present application, there are disclosed improved plastic containers and a method for their manufacture. In this method, the container is blow molded in the first hot mold which is maintained at 130-250.degree. C., the container is in contact with the mold surface for a short period of time (1-10 sec.) which is sufficient to induce partial crystallization. The container is then maintained at lower internal pressure to prevent significant shrinkage and then the container is rapidly transferred into a cold mold having substantially the same volume or smaller volume than the hot mold. The temperature of the cold mold is maintained at 1-100.degree. C. The container is quenched in the cold mold. The method results in a thermally stable container which has higher onset-of-shrinkage temperature and higher mechanical properties as required for hot fill applications. The method also provides lower cycle time.
Where the base of the container is complex such as having axially projecting portions so that it is free standing or has an inverted bottom, transfer from a hot mold to a cold mold, while the interior of the container is under pressure, tends to deform the bottom and cause it to revert to a hemispherical bottom. This tends to occur both when the blown container is maintained stationary and when the container is moved from a hot mold to a cold mold.
To overcome these problems in copending application Ser. No. 021,894, filed Mar. 4, 1987, now U.S. Pat. No. 4,839,127, having a common assignee with the present application, there is disclosed a method for making a partially crystalline, biaxially oriented heat set hollow plastic container from a hollow parison having an open end and a closed end comprising engaging the open end of a plastic parison which is at a temperature within its molecular orientation temperature range, positioning a mold base in axial alignment with said engaged hot parison, enclosing a hot mold about the mold base, the mold being at neat setting temperature, expanding the plastic parison within the hot mold and mold base by internal pressurization to induce biaxial orientation of the plastic parison and force the plastic parison into intimate contact and conformance with the hot mold and to maintain contact by such internal pressurization between the mold and the biaxially oriented container for a time sufficient to induce partial crystallization in the biaxially oriented container, reducing the internal pressure in the blown container, opening the hot mold while maintaining engagement of the open end and engagement of the mold base with the blown hollow container, maintaining a lower internal pressurization of the container to prevent significant shrinkage, and cooling the container while maintaining engagement of the open end and engagement of the mold base with the container and maintaining pressure in the container at least sufficient to prevent shrinkage.
In copending application Ser. No. 020,830, filed Mar. 2, 1987, and now abandoned, having a common assignee with the present application, there is disclosed a method for making a partially crystalline, biaxially oriented heat set hollow polyethylene terephthalate free standing container from a hollow parison having an open end and a closed end comprising engaging the open end of a plastic parison which is at a temperature within its molecular orientation temperature range, positioning a mold base in axial alignment with said engaged hot parison, enclosing a hot mold about the mold base, the mold being at heat setting temperature and the mold base being at a temperature preferably significantly lower than the mold, expanding the plastic parison within the hot mold and mold base by internal pressurization to induce biaxial orientation of the plastic parison and force the plastic parison into intimate contact and conformance with the hot mold and to maintain contact by such internal pressurization between the mold and mold base and the biaxially oriented container for a time sufficient to induce partial crystallization in the side wall and base of the biaxially oriented container, reducing the internal pressure in the blown container, opening the hot mold while maintaining engagement of the open end of the blown hollow container, disengaging the hot mold base from the blown container, maintaining a lower internal pressurization of the container to prevent significant shrinkage, and engaging a cold mold base with the base of the container, and enclosing a cold mold about the container and the mold base to cool the container while maintaining engagement of the open end with the container and maintaining pressure in the container at least sufficient to prevent shrinkage, and opening the cold mold and releasing the container.
In copending application Ser. No. 020,813, filed Mar. 2, 1987, and now abandoned, having a common assignee with the present application, there is disclosed a method and apparatus of forming a hollow partially crystalline heat set biaxially oriented polyethylene terephthalate container having a hemispherical base comprises blowing heated parison at orientation temperature outwardly against the confines of a mold which is at heat setting temperature wherein the side wall and the major portion of the base of the container are subjected to a heat setting temperature and the center of the base of the container is subjected to a temperature not greater than 150.degree. C. so that the central portion is crystallized substantially lesser amount than the side wall and the major portion of the base. The container is then quenched. The resultant container can be formed at a rapid cycle and has good drop impact properties and obviates the problems of sticking to the hot mold or blow out when the hot mold is opened.
Although the methods set forth in the aforementioned applications Ser. Nos. 923,503, 021,894, 020,830, and 020,813 result in substantially less cycle times, the transfer of blown containers from the hot mold to the cold mold is by relative movement after the molds are opened and therefore is very critical. Care must be exercised to prevent distortion of the containers during transfer. Therefore, sufficient time must be allotted to transfer in order to prevent distortion and this limits the cycle time. It is desirable or commercial purposes to further decrease the cycle times. In addition, it is desirable to provide an apparatus which will commercially produce satisfactory containers at the minimum cycle times.
Accordingly among the objectives of the present invention are to provide a method and apparatus for making partially crystalline, biaxially oriented heat set hollow plastic containers at faster cycle times; which method and apparatus can be utilized to provide faster cycle times in accordance with any of the above-described methods which utilize a hot mold and a cold mold; to provide an apparatus which will make such containers in commercial quantities; to provide an apparatus which is capable of being selectively changed to perform each of a variety of methods; to provide an apparatus having a novel stretch and blow apparatus, and to provide an apparatus having a novel parison handling apparatus.
In accordance with the present invention, the method and apparatus for making a partially crystalline, biaxially oriented heat set hollow plastic container from a hollow parison of crystallizable material having an open end and a closed end comprises engaging the open end of a first plastic parison which is at a temperature within its molecular orientation temperature range, enclosing a hot mold about the parison at a first position, the mold being at heat setting temperature, simultaneously enclosing a cold mold at a second position about a previously blown container in the hot mold to cool the container, expanding the plastic parison within the not mold by internal pressurization through the open end to induce biaxial orientation of the plastic parison and force the plastic parison into intimate contact and conformance with the hot mold and to maintain contact by such internal pressurization between the mold and biaxially oriented container for a time sufficient to induce partial crystallization in the biaxially oriented container, moving the hot mold and the cold mold in unison to bring the hot mold to the second position and the cold mold to a third position during the expanding of the parison within the hot mold and during the cooling of the previously blown container in the cold mold, maintaining a lower internal pressurization of the container in the hot mold and exhausting the pressurization to atmospheric pressure in the cold mold, substantially simultaneously opening the hot mold add cold mold, moving the open hot mold to the first position and open cold mold to the second position, and enclosing the hot mold about a second plastic parison which is at a temperature within its molecular orientation temperature range, and enclosing the cold mold about a previously blown container from the first parison.
The method and apparatus can be adapted to the aforementioned method of U.S. Pat. No. 4,522,779 and the aforementioned patent applications Ser. Nos. 923,503, 021,894, 020,830 and 020,813 which are incorporated herein by reference.